Reference can be made to our U.S. Pat. No. 4,359,644 which described an improved form of ripple control in which a transmitter generated audio frequency signals on a power system as a sequence of waveform distortions by means of a series connected inductive-capacitive oscillating load, the signal being detected in receivers using a synchronous correlation method of detection in which two synchronous filters (correlators) were run in quadrature and their squared outputs summed to form a non-phase sensitive detector.
The main object of this invention is to provide improvements to the SWD control scheme, as described in aforesaid U.S. Pat. No. 4,359,644, which can show financial savings while retaining an even narrower bandwidth than that achieved in devices made according to the specification of that patent, and to provide modifications to the invention disclosed in U.S. Pat. No. 4,868,539.
In the SWD control scheme described in U.S. Pat. No. 4,359,644 aforesaid identical control frequencies were generated in both the transmitter and the receiver. These two control frequencies are made identical by using the 60 Hz power mains frequency as a reference, synthesising a higher frequency using a phase looked-loop frequency multiplier circuit and then dividing by an integer. In this way, by using different dividing integers, a whole range of control frequencies were generated which were always identical at each end, that is, at the transmitter end and the receiver end of a power line. The control frequency was used in the transmitter to control the frequency of the inductive-capacitive oscillating load and thus the signal produced on the power system. In the receiver the control frequency was used to drive the synchronous filters and thus determine the centre frequency for signal detection. The frequency of the output from each synchronous filter was equal to the difference between the signal frequency and the synchronous filter frequency. Hence when these two were identical the synchronous filter outputs were unidirectional or DC voltages. By measuring the time between zero crossings of these outputs it could be determined if these outputs were DC voltages or low frequency AC voltages. If these output frequencies were greater than a predetermined limit the receiver could be programmed to ignore the received signal. In this way discrimination could be achieved between signals as closely spaced as 0.5 Hz apart and thus a multi-frequency system of coding can be used.